A field study in western Serbia evaluated during two seasons (from 2005 to 2007) the effect of nitrogen (N) fertilizer application on Italian ryegrass (Lolium italicum Lam.) grown for seed and its competitive interaction with cleavers (Galium aparine L.) in the year after establishment. Increasing rates of N fertilizer (control, N-1-50, N-2-100 and N-3-150 kg ha(-1) N) and two ryegrass sowing rates (5 and 20 kg ha(-1)) were used. The increase of fertilizer N level improved competitive ability of ryegrass and affected the interspecific competitive interactions. The interaction between Italian ryegrass and cleavers was evaluated by measuring total nitrogen content (TNC) in the leaf during vegetation and seed yield at the end of vegetation period. The highest seed yield was obtained with 100 kg ha(-1) N in both experimental years as well as in ryegrass monoculture and ryegrass-cleavers mixture. Competition with cleavers decreased ryegrass seed yield.